High efficient power electronic devices (also referred to as power switch devices) are of significant application value in domains such as smart grids, industrial controlling, new energy power generation, electric vehicles, and consumer electronics, etc. Globally, more than 70% of power electronic systems are manipulated and administrated by power administration systems based on power semiconductor devices. Performance of conventional Si power electronic devices approaches to the physical limit of Si semiconductor materials. Novel type of wide forbidden-band semiconductor devices, such as SiC and GaN, have higher breakdown electric field value, higher operation frequency and even lower on-resistance, and thus have already become promising candidates for the next generation of high efficient power electronics.
Enhancement-mode is essential for safe operation of power electronic devices, which ensures safety of the devices even without gate control when it is operated under high voltage, and will not cause damages of the system. For this reason, the power electronic devices have to be enhancement-mode (also referred to as normally-off) devices, that is, thresholds for the devices must be above 0V. Currently, GaN-based enhancement-mode power electronic devices are mainly manufactured on the basis of Al(In,Ga)N/GaN heterogeneous structures, in which, relying on strong spontaneous piezoelectric polarization effect between Al(In,Ga)N barrier layers and GaN buffer layers, a two-dimensional electron gas (2 DEG) with a density of up to 1013 cm−2 will be induced in channels of Al(In,Ga)N/GaN heterostructures. Therefore, GaN-based power electronic devices which are manufactured based on such structures (including HEMTs and MIS-HEMTs) are generally depletion-type. Several kinds of techniques are world-widely used to realize GaN-based enhancement-mode devices, mainly comprising: 1) thinning the Al(In,Ga)N barrier layer by gate trench etching; 2) injecting negative fluoride ions into the Al(In,Ga)N barrier layer; 3) growing a P—(Al)GaN cap layer on surface of the barrier layer; 4) growing a InGaN or thick GaN anti-polarization layer on surface of the barrier layer; 5) a cascode configuration of enhancement-mode Si-MOSFET and GaN-based depleted HEMT/MIS-HEMT.
Gate trench etching is achieved by etching Al(In,Ga)N barrier layer with a plasma dry etching process. Since thickness of the barrier layer is typically about 20 nm, uniformity of etching depth among different wafers, especially those from different batches, are difficult to be maintained using such a technique, and industrialization of this technique is restrained. The fluoride ion injecting technique meets similar issues. The P—(Al)GaN cap layer technique and thick GaN anti-polarization layer technique implements the enhancement-mode by controlling thickness and doping of Metal Organic Chemical Vapor Deposition (MOCVD) or molecular beam epitaxy (MBE) growth, which in general may obtain a fine threshold uniformity and has been reported an exemplary product for P—(Al)GaN technique. The cascode technique uses well-developed Si-MOSFET (already industrialized) to implement the enhancement-mode, and exemplary power electronic products of 600V have been proposed.
In addition, because of the presence of the surface state, GaN-based power electronic device may exhibit serious current collapse phenomenon when it is operated at high voltage, which directly leads to increase of dynamic on-resistance and power consumption of the device. Researchers from Hong Kong University of Science and Technology use Plasma Enhancement-mode Atom Layer Deposition (PEALD) technique to epitaxial grow an AlN film with polarization characteristics on an III-nitride semiconductor. In this research, polarization-induced high density polarized charges are used to compensate for the surface state, such that the current collapse phenomenon at high voltage may be inhibited for GaN-based power electronic devices. Similarly, researchers from Institute of Microelectronics of Chinese Academy of Sciences also found that high density of positive fixed charges can also be induced by a SiNx passivation layer on the surface of GaN-based heterostructures, and effective suppression of current collapse in GaN power devices can also be achieved with SiNx grown by low-pressure chemical vapor deposition (LPCVD). Therefore, by utilizing the polarization characteristics of polarized AlN film or SiNx layer with positive fixed charges, besides inhibiting current collapse, 2 DEG of high density may also be achieved in thin barrier Al(In,Ga)N/GaN heterostructure.
In conclusion, combination of good enhancement-mode threshold control of the thin barrier Al(In,Ga)N/GaN heterostructure and high-density positive charges in polarized-AlN or SiNx passivation film helps to manufacture GaN-based power electronic devices with fine enhancement-mode threshold uniformity and low dynamic on-resistance, so that process repeatability and production yield of the GaN-based enhancement-mode device may be effectively improved, and industrialization of the GaN-based electronic devices are further pushed forward.